[Relay][Text Format] Pretty Printer Smart Inlining

[joshpoll]

• Adds smart inlining. Attempts to inline a node at the end of a scope or in a let value position when it is unique.
• Removes GNF flag and debug printer. Smart inlining provides the benefits that turning GNF off provided, but without losing information about the original program.
• Refactors dependency graph code out of to_a_normal_form.cc.

cc @vinx13 @wweic @MarisaKirisame @tqchen

[MarisaKirisame]

Make these two member of the language instead of passing them around.
If you pass them around you should change them. If they are constant save yourself the work.

[joshpoll]

They are flags for the Print function and are changed by the callers in specific instances. They are only changed for one invocation of Print at a time and importantly not for recursive invocations. I think they make the most sense as flags on Print rather than as instance variables.

[wweic]

lgtm.

[tqchen]

@joshpoll Please clearly document the start inliner's behavior. Some suggestions:

• Have a max depth which can be inlined
• Have a white list of ops that can be inlined, set by op_attr, this way we don't mistakenly inline conv into another conv2d

[tqchen]

@joshpoll please update this PR

[joshpoll]

@tqchen I have updated the PR documenting the inline heuristics. I don't think it's a good idea to add more until we have a better doc model that gives more layout options.

[tqchen]

Thanks, @joshpoll @MarisaKirisame @wweic this is now merged